President of the Australian Catholic Bishops Conference Archbishop Francis Carroll has voiced his support for yesterday's statement from Prime Minister John Howard that ruled out any moves from the Federal Government to legalise same-sex marriage.

Comments from the Prime Minister came in the context of debate that followed last week's release of the instruction from the Vatican's Congregation for the Doctrine of the Faith condemning gay marriage and advising politicians of their moral duty to stand in the way of attempts to legalise same-sex unions.

Archbishop Carroll said: "We warmly welcome Prime Minister Howard's strong support of marriage as a lifelong union between a man and a woman, which forms a fundamental institution in our society and is the foundation of family life."

Howard said recognition of gay marriages would weaken the institution of marriage.

"Traditional marriage is one of the bedrock institutions of our society and I don't want anything to occur that further weakens it," he said. "Marriage, as we understand it in our society, is about children, having children, raising them, providing for the survival of the species."

Treasurer Peter Costello acknowledged the reality of long-term same-sex relations, but backed Mr Howard's stand in his refusal to countenance a change in legislation.

Neither politician bought into the contentious question of the Vatican's right to intervene in the democratic and party political process by requiring politicians to oppose gay marriage.

Prominent Catholic MPs, including Labor's Laurie Brereton and Liberal Christopher Pyne, responded last week to the church's statement by insisting on a firm separation of church and state. Melbourne Auxiliary Bishop Christopher Prowse said the Catholic Church sympathised with politicians struggling to reconcile the demands of church and party. He said in cases of conflict, politicians should simply do their best.